The primary goal has been the elucidation of the structures of reactive metabolites responsible for the carcinogenic, cytotoxic and mutagenic activity of drugs, polycyclic aromatic hydrocarbons, and other environmental chemicals. The approach taken consists of: i) synthesis of primary and secondary oxidative metabolites, ii) study of the metabolism of the chemicals with liver microsomes and with purified cytochromes P450 and epoxide hydrolase, iii) evaluation of the mutagenicity and tumorigenicity of the synthetic metabolites, iv) elucidation of the roles of the cytochrome P450 system and epoxide hydrolase in modulating the mutagenicity of these metabolites, v) determination of the rates and products of reactions of arene oxides and diol epoxides with biopolymers and model compounds, and vi) search for agents capable of preventing the tumorigenicity of reactive metabolites. Most of the effort in the past year has concentrated on DNA adducts of carcinogenic bay-region diol epoxide metabolites. A strategy has been developed for synthesis of oligonucleotides containing N6-2'-deoxyadenosine and N2-2'-deoxyguanosine adducts of diol epoxides where the exocyclic amino groups of the purine bases are bonded to the hydrocarbon by cis or trans opening of the epoxide at the benzylic position. The required N-substituted purines are prepared from hydrocarbon aminotriols and appropriately blocked fluorine analogs of the bases. The blocking groups were selected such that standard phosphoramidite chemistry can be used on an automated DNA synthesizer. Such adducted DNA oligomers are expected to be of immense value in the study of mechanisms of mutagenesis and carcinogenesis. Mutations in the coding region of the hypoxanthine (guanine) phosphoribosyltransferase (HPRT) gene of Chinese hamster V-79 cells were examined with the car- cinogen (+)-7R,8S-dihydroxy-9S,10R-epoxy-7,8,9,10-tetrahydrobenzo[a]- pyrene. Although this carcinogen is known for causing point mutations at guanine, studies at very low, noncytotoxic doses have indicated that comparable numbers of mutations also occur at adenine, suggestive that there is a small number of A-T hot spots which are obscured by mutations at G-C base pairs when higher, nonenvironmental doses are used.